Diffusion-weighted parallel imaging with navigator-signal-based phase correction

ABSTRACT

A magnetic resonance imaging method for forming an image of an object from a plurality of signals acquired by an array of multiple receiver antennae, wherein spins are excitated in a part of the object. MR signals are measured along a predetermined trajectory containing a plurality of lines in k-space by application of a read gradient and other gradients. Further, a navigator gradient is applied for the measurement of navigator MR signals and an additional gradient is applied in order to achieve diffusion sensitivity of the MR signal, wherein phase corrections are determined from phases and moduli of the navigator MR signals so as to correct the measured MR signals. An image of the part of the object is determined from the corrected MR signals. The corrected phase is determined from the weighted phase difference between a reference navigator signal for each antenna and the actual navigator MR signal of said antenna.

The invention relates to a magnetic resonance (MR) method for theimaging of an object arranged in a steady magnetic field, whereas thefollowing steps being repeatedly executed according to said method:

-   -   excitation of spins in a part of the object,    -   measurement of MR signals along a predetermined trajectory        containing a plurality of lines in k-space by application of a        read gradient and other gradients,    -   application of a navigator gradient for the measurement of        navigator MR-signals,    -   said method also including the determination of a phase        correction from phases and moduli of the measured navigator MR        signals so as to correct the measured MR signals and the        determination of an image of the part of the object from the        corrected MR signals.

The invention also relates to an MR device for carrying out such amethod.

A method of the kind set forth is known from WO-A-98/47015, in which themethod is applied to the specific case of diffusion weighted imaging.Here, a corrected phase is determined for a measured navigator MR signalfrom a measuring point, for which the modulus of the measured navigatorMR signal is smaller than a threshold value, from the phases of themeasured navigator MR signal form different reference measuring pointsfor which the moduli of the navigator MR signal exceed the thresholdvalue. The method is based on the fact that the presence of a strongdiffusion motion due to a high value of the additional gradient reducesthe value of the moduli of the measuring points in the navigator MRsignal which correspond to regions of the brain which contain a largequantity of cerebrospinal fluid (CSF). Because of the low value of themoduli, the error in the determination of the phase increases. With thedescribed method the artefacts in the MR image can be reduced when forthe measuring points having a modulus smaller than the threshold valuethe corrected phase is determined from the phases of the variousreference measuring points of the navigator MR signal for which thephase can be determined with a sufficiently small error.

In magnetic resonance imaging there is a general tendency to obtainacceptable images within shorter periods of time. For this reason thesensitivity encoding method called “SENSE” has recently been developedby the Institute of Biomedical Engineering and Medical Informations,University and ETH Zürich, Switzerland. The SENSE method is based on analgorithm which acts directly on the image as detected by the coils ofthe magnetic resonance apparatus and which subsequent encoding steps canbe skipped and hence an acceleration of the signal acquisition forimaging by a factor of from two to three can be obtained. Crucial forthe SENSE method is the knowledge of the sensitivity of the coils whichare arranged in so called sensitivity maps. In order to accelerate thismethod there are proposals to use raw sensitivity maps which can beobtained through division by either the “sum-of-squares” of the singlecoil references or by an optional body coil reference (see e.g. KPruessmann et. al. in Proc. ISMRM, 1998, abstracts pp. 579, 799, 803 and2087). In fact the SENSE method allows for a decrease in scan time bydeliberately undersampling k-space, i.e. deliberately selecting aField-of-View (FOV) that is smaller than the object to be acquired. Fromthis undersampling fold-over artefacts are obtained which can beresolved or unfolded by the use of the knowledge of a set of distinctcoils having different coil sensitivity patterns. The undersampling canbe in either one of both phase-encoding directions.

According to the first mentioned method the phase navigator signals aremeasured per single coil element of an array of multiple receiver coils,i.e. with the same coil element as was used for imaging. Also phasecorrection is applied per single coil. This way of correction can havetwo unwanted consequences:

1. if one needs the phase relation between the coils or synergychannels, as necessary for instance with the SENSE method, the correctedsignals can be disturbed by a difference between the phase correctionsper channel,

2. in regions where the coils or synergy channels measure a too lowsignal, the correction will be applied with high noise, which destroysphase encoding accuracy, resulting in many artefacts in the imageregion. In FIGS. 1 a and 1 b the modulus of the gradient signal m inx-direction and the related phase correction signal Φ is given for twosynergy elements S₁ and S₂. The direction x of the measurement isrunning from element S₁ to S₂. In the region A between both dashed lines100 and 101 the phase correction may cause problems because of a highnoise level.

It is thus an object of the present invention to prevent aliasing indiffusion-weighted MR imaging.

This object of the invention is achieved by a method as defined in claim1. The invention is further related to an apparatus as defined in claim6 and to a computer program product as defined in claim 7.

These and other advantages of the invention are disclosed in thedependent claims and in the following description in which anexemplified embodiment of the invention is described with respect to theaccompanying drawings. Therein, FIG. 2 shows an MR device which includesa first magnet system 2 for generating a steady magnetic field, and alsomeans for generating additional magnetic fields having a gradient in theX, Y, Z directions, which means are known as gradient coils 3. The Zdirection of the co-ordinate system shown corresponds to the directionof the steady magnetic field in the magnet system 2 by convention. Themeasuring co-ordinate system x, y, z to be used can be chosenindependently of the X, Y, Z system shown in FIG. 2. The gradient coilsor antennae are fed by a power supply unit 4. An RF transmitter coil 5serves to generate RF magnetic fields and is connected to an RFtransmitter and modulator 6. A receiver coil is used to receive themagnetic resonance signal generated by the RF field in the object 7 tobe examined, for example a human or animal body. This coil may be thesame coil as the RF transmitter coil 5 or an array of multiple receiverantennae (not shown). The coil 5 is a non phased-array receiver antenna,which is different from the array of multiple receiver antennae.Furthermore, the magnet system 2 encloses an examination space which islarge enough to accommodate a part of the body 7 to be examined. The RFcoil 5 is arranged around or on the part of the body 7 to be examined inthis examination space. The RF transmitter coil 5 is connected to asignal amplifier and demodulation unit 10 via a transmission/receptioncircuit 9. The control unit 11 controls the RF transmitter and modulator6 and the power supply unit 4 so as to generate special pulse sequenceswhich contain RF pulses and gradients. The control unit 11 also controlsdetection of the MR signal(s), whose phase and amplitude obtained fromthe demodulation unit 10 are applied to a processing unit 12. Thecontrol unit 11 and the respective receiver coils 3 and 5 are equippedwith control means to enable switching between their detection pathwayson a sub-repetition time basis (i.e. typically less than 10 ms). Thesemeans comprise inter alia a current/voltage stabilization unit to ensurereliable phase behavior of the antennae, and one or more switches andanalogue-to-digital converters in the signal path between coil andprocessing unit 12. The processing unit 12 processes the presentedsignal values so as to form an image by transformation. This image canbe visualized, for example by means of a monitor 13.

The invention will be described hereinafter, by way of example, on thebasis of versions of a method in which diffusion weighting is used incombination with a known echo planar imaging (EPI) pulse sequence so asto generate an MR signal. These EPI pulse sequences can be used to forman image by means of a two-dimensional or three-dimensional Fourierimaging technique. Another imaging technique for use of the presentinvention is SENSE as described in more detail in the above mentionedarticle of K. Pruesmann et. al.

The gist of the present invention is the use of a common (shared)correction vector for data of each separate coil or synergy channel.This common vector can be obtained from a data acquisition employing adifferent, volume encompassing coil or it can be derived as the weightedphase difference between a reference navigator acquisition and an actualnavigator acquisition using the array of multiple receiver antennae. Theweighting factor can either be the modulus of the reference navigatorsignal or can be the modulus of the not diffusion weighted signal atb=0.

Mathematically, both methods can be described as follows:$\begin{matrix}{\text{Method~~1:}\quad} \\{{q(x)} = {\sum\limits_{i}\frac{{n(x)}_{R,i}^{*} \cdot {n(x)}_{a,i}}{{n(x)}_{a,i}}}}\end{matrix}$whereas n(x)_(R,i) is the reference navigator signal in the hybrid space(x, k_(y)) for coil i, and n(x)_(a,t) is the actual navigator signal inthe hybrid space (x, k_(y)) with k_(y)=0 for coil i. In this case is$\Delta = \frac{q(x)}{{q(x)}}$the correction vector. $\begin{matrix}{\text{Method~~2:}\quad} \\{{q(x)} = {\sum\limits_{i}{\frac{{n(x)}_{R,i}^{*} \cdot {n(x)}_{a,i}}{{{n(x)}_{R,i}} \cdot {{n(x)}_{a,i}}} \cdot {{n(x)}_{{b = 0},i}}}}}\end{matrix}$This means that the modulus of n(x)_(i) at b=0 for each coil i is theweighting factor. Here also the correction vector is$\Delta = \frac{q(x)}{{q(x)}}$

1. A magnetic resonance imaging method for forming an image of an objectfrom a plurality of signals acquired by an array of multiple receiverantennae, wherein spins are excitated in a part of the object, MRsignals are measured along a predetermined trajectory containing aplurality of lines in k-space by application of a read gradient andother gradients, a navigator gradient is applied for the measurement ofnavigator MR signals, wherein phase corrections are determined fromphases and moduli of the navigator MR signals so as to correct themeasured MR signals and an image of the part of the object is determinedfrom the corrected MR signals, characterized in that a common correctionvector is used for correction of data from all receiver antennae of thearray.
 2. A method as claimed in claim 1, characterized in that thecommon correction vector is determined from the weighted phasedifference between a reference navigator signal for each antenna and theactual navigator MR signal of said antenna.
 3. A method as claimed inclaim 1, characterized in that the common correction vector is acquiredfrom a non phased-array receiver antenna, different from the array ofmultiple receiver antennae being used for MR image data acquisition. 4.A method as claimed in claim 2, characterized in that the weightingfactor is the modulus of the reference navigator signals.
 5. A method asclaimed in claim 2, characterized in that an additional gradient isapplied to generate diffusion weighting and that the weighting factor isthe modulus of the navigator signal without diffusion weighting.
 6. Amagnetic resonance imaging apparatus for obtaining an MR image from aplurality of signals comprising: means for excitation of spins in apartof the object, means for measuring MR signals along a predeterminedtrajectory containing a plurality of lines in k-space by application ofa read gradient and other gradients, means for applying a navigatorgradient for the measurement of navigator MR signals and an additionalgradient is applied in order to achieve diffusion sensitivity of the MRsignal, wherein phase corrections are determined from phases and moduliof the navigator MR signals so as to correct the measured MR signals andan image of the part of the object is determined from the corrected MRsignals, and means for applying a common correction vector, which isused for correction of data from all receiver antennae of the array. 7.An apparatus as claimed in claim 6, characterized in that means areprovided for determining the common correction vector from the weightedphase difference between a reference navigator for each antenna and theactual navigator MR signal of said antenna.
 8. An apparatus as claimedin claim 6, characterized in that means are provided for acquiring thecommon correction vector from a non phased-array receiver antenna,different from the array of multiple receiver antennae being used for MRimage data acquisition, further containing means for reliably switchingbetween acquisition with the non phased-array antenna and acquisitionwith the array of multiple receiver antennae on a sub-repetition timebasis.
 9. A computer program product stored on a computer usable mediumfor forming an image by means of the magnetic resonance method,comprising a computer readable program means for causing the computer tocontrol the execution of: excitation of spins in a part of the object,measuring of MR signals along a predetermined trajectory containing aplurality of lines in k-space by application of a read gradient andother gradients, applying a navigator gradient for the measurement ofnavigator MR signals, wherein phase corrections are determined fromphases and moduli of the navigator MR signals so as to correct themeasured MR signals and an image of the part of the object is determinedfrom the corrected MR signals, using a common correction vector forcorrection of data from all receiver antennae of the array.
 10. Acomputer program product as claimed in claim 9, wherein in addition tothe navigator gradient a reference navigator gradient is applied inorder to achieve diffusion sensitivity of the MR signal, and thecorrected phase is determined from the weighted phase difference betweenthe reference navigator signal for each antenna and the actual navigatorMR signal of said antenna.